Full Ending
There is a more complete version of the ending. To see it,
protect Kyrie from being damage for 1:30 seconds while the
credits roll. If successful, you'll get the full remainder of the
ending. You can re-fight the las boos and retry this as many
times.

Extra Difficulty Levels
Unlock Son of Sparda. Play through it to unlock Dante Must Die
and Heaven or Hell. Play through either mode and you get Hell or
Hell.